Trying to flip pose along x-axis does nothing
Maybe your problem is the following one:
Paste Pose Flipped will paste the pose you've copied to the opposite side.
But let say you copy the left upper arm bone pose only, to paste flip it to the right upper arm bone. The problem is that the right upper arm is part of the right lower arm IK chain, so, whatever pose you’ll try to give it, it will obey to the lower arm IK's target. That's probably why you think sometimes paste flip doesn’t work.
If you select the left IK controller (called HandIK.L) and paste flip it, it will move the whole right arm, this time the copy paste flip is visible because you did it on a bone that controls and not obeys.
If you select all the bones and copy paste flip them, it will paste flip all the bones poses, so it will paste flip left IK controllers to the right side, and right IK controllers to the left side, and all other bones as well, it will completely invert the pose.